1. Field of the Invention
The present invention relates to an aqueous emulsion, and more particularly to an aqueous emulsion which is excellent in high speed application properties, water-resistant adhesion properties and low-temperature stability.
2. Description of the Prior Art
Polyvinyl alcohol (hereinafter referred to as PVA) has heretofore been commonly used as a protective colloid for emulsion polymerization of ethylenically unsaturated monomers, particularly vinyl esters such as vinyl acetate. Aqueous emulsion containing as the dispersoid polyvinyl ester which is obtained in emulsion polymerization using PVA as the protective colloid has a very wide range of use as an adhesive for paper, wood and plastics; as a binder for impregnated paper, non-woven fabric product; and for many other uses such as mortar additive, mortar interlaminar bonding binder, and paints and in paper and fiber processing.
PVAs used as a dispersant for emulsion polymerization are classified into a "completely hydrolyzed PVA" with the degree of hydrolysis of approximately 98 mol % and a "partially hydrolyzed PVA" with the degree of hydrolysis of approximately 88 mol %. Emulsions obtained in emulsion polymerization using the completely hydrolyzed PVA as dispersant have relatively high water resistance and flowability (high speed application property). Aqueous emulsions of this type are, however, defective in that their viscosity radically increases when left standing at low temperatures and that they are easily subject to gelation. Emulsions obtained in emulsion polymerization using the partially hydrolyzed PVA as dispersant, on the other hand, are improved in terms of viscosity increase that may occur when left standing at low temperatures or in terms of gelation tendency, but they are inferior in water resistance when used as a coating agent and in flowability (including viscosity fluctuation due to changes in the application speed and splashing of the liquid at higher application speed).
In order to obviate the problems encountered in the prior art, there have been proposed combined use of PVAs with different degrees of hydrolysis or use of PVA of which degree of hydrolysis is intermediate between completely hydrolyzed and partially hydrolyzed. However, there is no PVA that satisfies various requirements all at once, such as water resistance as a coating material, high speed application property, less temperature-dependent emulsion viscosity and low-temperature stability. Addition of urea resin and various cross-linking agents to emulsions obtained by emulsion polymerization using partially hydrolyzed PVA as the dispersant has been proposed to improve the water resistance of the resultant coating. This improves the water resistance of the coating to a certain extent but entails a problem of inferior work efficiency due to increased viscosity. It is also known to use a dispersant of modified PVA containing 1 to 10 mol % of .alpha.-olein unit with a number of carbon atoms of 4 or less in order to improve the water resistance of the coating and the less temperature-dependent emulsion viscosity. However, an aqueous emulsion which is excellent both in water resistance and flowability (high speed application property) has never been made available (JPA Laid-open No. 80709, 1994).
It is therefore an object of the present invention to provide an aqueous emulsion with excellent flowability (high speed application property), water resistant adhesion property and low-temperature stability while maintaining the advantageous features of an aqueous emulsion containing PVA as protective colloid such as high viscosity, excellent mechanical stability, excellent work efficiency and high initial adhesive strength.
Another object of the present invention is to provide an adhesive with particularly good water resistance. Still another object is to provide a dispersant for emulsion polymerization for producing said aqueous emulsion having excellent physical properties.